1. Field of the Invention
The present invention relates to a mobile communication system and a hand-off method used in the mobile communication system.
2. Description of the Related Art
An electric field level measurement at a mobile terminal and a hand-off method in accordance with the result of the measurement carried out in a mobile communication system will be explained with referring to FIG. 2 which shows an example of constitution of the mobile communication system. A description will be made for the mobile communication system designed based upon the TIA/EIA-627 (telecommunications industry association/electronic industry association) specifications.
In FIG. 2, the mobile communication system designed based on the TIA/EIA-627 specifications corresponds to a cellular type mobile communication system. A communication service area by the cellular type mobile communication system comprises a plurality of cells 20, 30, 40, 50, 60, 70 and 80. Base stations 2, 3, 4, 5, 6, 7 and 8 are arranged in each of the cells 20, 30, 40, 50, 60, 70 and 80 for providing radio communication with mobile terminals existing in each cell. In this drawing, only 7 cells are shown, however, in an actual case, a large number of cells are constituting communication service area. Control channels having different frequencies from each other are allocated to the adjoining base stations. Each of these base stations 2, 3, 4, 5, 6, 7 and 8 is connected to a mobile switching center 1 by way of transmission lines including a control signal line and a communication line. In this mobile communication system, there are plural mobile terminals 9a to 9m freely moving through the above-explained cell groups.
Now, a mobile terminal 9a is located in the cell 20, and is in a standby state while being tuned to a forward control channel transmitted from the base station 2, and waiting for a call origination by a user of this mobile terminal or a call termination from the base station. At this time, when this mobile terminal 9a executes either a call originating operation or a call receiving operation, an idle communication channel of the base station 2 is allocated for the call by a control operation of the mobile switching center 1 via the transmission line 10, so that a communication service may be provided.
When the communication service is commenced, the base station 2 edits an electric field level measurement request signal obtained from a forward control channel list of the adjoining base stations provided in this base station 2. Then, this base station 2 transmits the electric field level measurement request signal to the mobile terminal 9a. In other words, the radio signal frequencies of the control channels which are allocated to the respective cells 30 and 40 of the base stations 3 and 4 located adjacent to the base station 2 are contained in the electric field level measurement request signal. The resulting electric field level measurement request signal is notified to the mobile terminals 9a. 
While continuing the communication service, the mobile terminal 9a periodically measures electric field levels of the forward control channels of the adjoining base stations 3 and 4 by tuning respective radio signal frequencies notified by the received electric field level measurement request signal. Then, the mobile terminals 9a produces an electric level measurement result signal by editing the measured electric field levels of the forward control channels of the adjoining base stations 3 and 4 and an electric field level of the forward communication channel under use for the current communication with the base station 2. This electric field level measurement result signal is periodically transmitted to the base station 2 from the mobile terminal 9a. 
The base station 2 compares the electric field level of the forward communication channel, which is used in the current communication, included in the electric field level measurement result signal transmitted from the mobile terminal 9a with a predetermined threshold value. This threshold value is defined as a minimum electric field level value at which a mobile terminal can continue a communication with a base station in the cell.
When the electric field level measurement value of the forward communication channel used in the current communication becomes smaller than, or equal to the threshold value, namely, is deteriorated, the base station 2 judges that the mobile terminal 9a is located at a boundary area of the cell 20. Then, this base station 2 edits both this electric field level measurement value and the electric field level measurement values of the forward control channels of the adjoining cells 30 and 40 to thereby produce an electric field level deterioration report signal. The base station 2 transmits this electric field level deterioration report signal to the mobile switching center 1.
In response to the received electric field level deterioration report signal, the mobile switching center 1 refers to a hand-off list provided in the mobile switching center 1 so as to select such an adjoining cell that the electric field level measurement value of the forward control channel being the most optimum value, and executes the hand-off control in such a manner that the current communication by the mobile terminal 9a can be switched and continued by a base station in a selected cell.
In such a conventional mobile communication system, when the mobile terminal 9a, which had been receiving the communication service in the cell 20, has moved into the cell 30 while measuring the electric field levels of the forward control channels of the adjoining base stations 3 and 4 in accordance with the electric field level measurement request signal received from the base station 2, the mobile switching center 1 executes the hand-off control by allocating the idle communication channel available in the hand-off destination base station 3 for continuing the communication service by the mobile terminal 9a. 
On the other hand, while holding the forward control channel information of the adjoining base stations 3 and 4, which was instructed by the original hand-off base station 2, the mobile terminal 9a continues the periodical electric field level measurement even after the mobile terminal 9a has moved in the cell 30 of the hand-off destination base station 3, and then reports the electric field level measurement result signal to the hand-off destination base station 3. However, this electric field level measurement result signal is a useless signal with respect to the hand-off destination base station 3, which will transmit the new electric field level measurement request signal to be used in the cell 30.
At this time, it could happen that the hand-off destination base station 3 receives this useless electric field level measurement result signal before transmitting the new electric field level measurement request signal to the mobile terminal 9a so as to measure the electric field levels of the forward control channels of the adjacent base stations 2, 4, 5 and 6. If the electric field level value of the forward communication channel having been used for the communication in the cell 20 is contained in this useless electric field level measurement result signal, and the value indicates deterioration of the signal level of the forward communication channel, the hand-off destination base station 3 produces the electric field level deterioration report to the mobile switching center 1.
However, this electric field level deterioration report is produced in accordance with the wrong report from the mobile terminal 9a, in which the electric field level measurement request has not yet been renewed.
The mobile switching center 1 executes the hand-off control based on the electric field level deterioration report caused by the wrong report, and allocates the idle communication channel of the adjoining base station which is selected as the next hand-off destination base station in accordance with the electric field level measurement values of the forward control channels contained in the received electric field level deterioration report. However, the selected base station does not coincide with such a base station which is to be referred from the hand-off list provided in the mobile switching center 1. As a result, the erroneous hand-off operation is repeatedly carried out. Otherwise, the call being established is interrupted. Accordingly, the communication service cannot be effectively provided.
The electric field level measurement stop signal defined in the TIA/EIA-627 specifications can be applied to avoid above-mentioned problem. However, the following problems will still occur when such an additional signal is introduced. That is, it bear on the hand-off control processing capacity and time. The hand-off success rate with respect to the mobile terminal which is moving in a high speed is lowered, and also, due to increase of the control signals used in the base station, the processing capability is lowered.
An object of the present invention is to solve such conventional problems, and therefore, is to provide a mobile communication system capable of improving a hand-off success rate with respect to a mobile terminal moving in a high speed, and also capable of increasing a processing capability of a base station while executing a hand-off control.
A mobile communication system, according to the present invention, is featured by comprising a plurality of radio base stations installed in each of plurality of cells, for providing mobile communication services, a mobile switching center connected to the plurality of base stations, for controlling call connections in the mobile communication system, and a plurality of mobile terminals, each includes an electric field level measurement stopping unit which stops an electric level measurement of forward control channels of adjoining base stations when having received a hand-off instruction signal transmitted from the mobile switching center.
The base station also includes an electric field level measurement result discarding unit which discards an electric field level measurement result signal transmitted from the mobile terminal after the hand-off instruction signal having been transmitted to the mobile terminal.
According to the present invention, a hand-off method for a mobile communication system in which a hand-off control is performed based on a deterioration of an electric field level of a communication channel between a base station and a mobile terminal which has measured and reported electric filed levels of forward control channels of adjoining base stations around a cell and the communication channel, the method comprising a step of stopping measurement and reporting, in the mobile terminal, for the electric filed levels of forward control channels of adjoining base stations and the communication channel when a hand-off instruction to another base station is received, and a step of resuming measurement and reporting, in the mobile terminal, for the electric filed levels of forward control channels of new adjoining base stations and new communication channel in accordance with an instruction by new base station of a hand-off destination.
Also, according to the present invention, a hand-off method for a mobile communication system structured by a plurality of radio base stations installed in each of plurality of cells, for providing mobile communication services, a mobile switching center connected to the plurality of base stations, for controlling call connections in the mobile communication system, and a plurality of mobile terminals moving around the plurality of cells and communicating with the base station in each of cells, the method comprising;
(1) instructing the mobile terminal to measure and report each of electric field levels of forward control channels of adjoining base stations around a cell in which the mobile terminal is currently locating, and to measure and report an electric field level of a communication channel used for a current communication with the base station, when a communication between the base station and the mobile terminal is commenced;
(2) requesting a hand-off control to the mobile switching center from the base station when a deterioration of the electric field level of the communication channel having been detected by the electric field level measurement results reported by said mobile terminal;
(3) instructing the mobile terminal to perform a hand-off operation to a base station, whose electric field level of the control channel contained in the electric field level measurement results reported by the mobile terminal is the highest level;
(4) discarding the electric field level measurement results reported from the mobile terminal in said base station after the hand-off instruction having been transmitted to the mobile terminal; and
(5) stopping the electric field level measurement and report, having been performed in the mobile terminal, when the hand-off instruction is received from the mobile switching center.
The hand-off method further comprising a step of resuming the electric field level measurement and report, by the mobile terminal, of forward control channels of adjoining base stations around a cell in which the mobile terminal is newly locating, and a communication channel used for new communication with new base station of the hand-off destination, when an electric field level measurement request is instructed from the new base station.